Pfizer Inc. plans to pursue a bid for AstraZeneca PLC, according to people familiar with the matter, eyeing a tie-up that would create a roughly $300 billion pharmaceutical giant and fuel an already booming year for merger-and-acquisition activity, particularly in health care.

An announcement of New York-based Pfizer's intention to pursue a deal for its U.K. rival is imminent, the people said. With AstraZeneca's market capitalization...